Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 77

4.2 Historical earthquakes and their impact

Although well inside the North American plate and remote from plate boundaries, the SLRS
has been subject to damaging earthquakes larger than magnitude 1 5 (Lamontagne et al .,
2008a ; Cassidy et al ., 2010 ; Table 4.1 ) . For the pre-twentieth-century events, magnitudes
and locations are approximate due to the variable reliability of the sources of informa-
tion: felt reports, detection completeness, and geographic distribution of the population.
An example of this uncertainty is the debate over the magnitude of the 1663 Charlevoix
earthquake (generally estimated at M 7; see Bent [2009] for a review). More recently,
Ebel ( 2011 ) rates the same earthquake as moment magnitude ( M )7.5 0.45 based on

felt reports and reported falling of masonry. This is not just an academic debate, as it may
appear: the preferred magnitude impacts seismic hazard estimates. The locations of some
historical events are also uncertain. An example of this is the 1732 magnitude 5.8 Montreal
earthquake, located either near Montreal (Leblanc, 1981 ) or in northern New York State
( Figure 4.2 ; Gouin, 2001 ) . These uncertainties in magnitudes and locations of historical
earthquakes lead to discrepancies in the various earthquake catalogues (with impact on
seismotectonic interpretation and seismic hazards). An example is the correlation of the
1732 earthquake with the rift faults around Montreal (Adams and Basham, 1989 ) . For-
tunately, most instrumentally recorded twentieth-century earthquakes have better defined
magnitudes and epicentres (Bent, 2009 ) .
Historically, only a few St. Lawrence Valley earthquakes had a geological impact such as
rock falls, landslides, slumps, earth flows in clay deposits, ground cracking, lateral spread-
ing, and liquefaction. No surface rupture has ever been reported. The St. Lawrence Valley
( Figure 4.2 ) has thick sequences of post-glacial marine clay deposits and these have proven
to be highly susceptible to earth flow under ground shaking. Of all SLRS earthquakes, the
M 7 1663 earthquake had the largest geotechnical impact, causing landslides in the epicen-
tral region (Filion et al ., 1991 ) , and along the Saguenay and Saint-Maurice rivers, more than
200 km away (Legget and LaSalle, 1978 ; Desjardins, 1980 ) . The 1663 event may have also
produced a basin collapse in the Saguenay Fjord (Syvitski and Schafer, 1996 ) . Some sub-
marine landslides of the Saguenay and St. Lawrence rivers are also associated with some of
these large Charlevoix earthquakes. An example is a landslide near Betsiamites on the Que-
bec North Shore, possibly caused by the 1663 Charlevoix earthquake, which first occurred
on land and continued offshore beneath the St. Lawrence River (Cauchon-Voyer et al .,
2007 ) . Another notable earthquake is the 1870 M 6

±

CSZ earthquake ( Figure 4.2 ) , which
had some dramatic consequences, including the collapse of buildings, liquefaction, and
landslides in the epicentral region, and rock falls along the Saguenay River (Lamontagne
et al ., 2007 ) . A landslide, most probably linked with the earthquake, killed four people five
days after the mainshock (Lamontagne et al ., 2007 ) . More recently, the 1988 M 5.9 Sague-
nay earthquake ( Figure 4.2 ) exemplifies the potential impact of a moderate eastern Canadian

½

1 M is defined as the most widely accepted magnitude value (generally the moment magnitude or the felt area magnitude for
historical earthquakes) for a given event.